1. Field of the Invention
The present invention relates to power steering apparatuses for applying a steering assist force to a steering mechanism by hydraulic pressure created by a pump driven by an electric motor.
2. Description of Related Art
Conventionally, power steering apparatuses for assisting in operating a steering wheel by supplying working oil from an oil pump to a power cylinder coupled to a steering mechanism have been employed. In such power steering apparatuses, the oil pump is driven by an electric motor. A steering assist force corresponding to the rotational speed of the electric motor is produced from the power cylinder.
Drive control of the electric motor is achieved by an electronic control unit carrying out on-off control of a motor driving element composed of an FET (Field Effect Transistor). In some circumstances, the steering wheel continues to be violently operated, so that the motor driving element is frequently turned on and off. There are also circumstances that the load on the electric motor is increased, resulting in a large current flow in the motor. In these situations, the motor driving element generates heat. Accordingly, the motor driving element may be destroyed.
The electronic control unit is constituted by a computer including a CPU, a RAM, and a ROM, for example, and has low resistance to heat applied from the exterior. When the motor driving element generates heat because the steering wheel continues to be violently operated, therefore, the electronic control unit may be destroyed by the heat generation from the motor driving element.
In the conventional power steering apparatus, therefore, there is provided a temperature sensor for detecting the internal temperature of the electronic control unit, for example. If the temperature detected by the temperature sensor is higher than a predetermined temperature, the electronic motor is forced to be stopped. After the temperature detected by the temperature sensor is lowered to not more than the predetermined temperature, the electronic motor which has been forced to be stopped is restarted. Consequently, the motor driving element and the electronic control unit can be prevented from being destroyed by the heat generation.
In the above-mentioned conventional control, however, after the electric motor is forced to be stropped, a driver may have a feeling of physical disorder in steering when the internal temperature of the electronic control unit is lowered to not more than the predetermined temperature while the steering wheel is being operated. That is, in a case where the driver is applying torque to the steering wheel, when the internal temperature of the electronic control unit is lowered to not more than the predetermined temperature, the electric motor is restarted in response thereto, thereby suddenly assisting in steering the steering wheel. Accordingly, the driver feels that the steering resistance is rapidly lowered.
An object of the present invention is to provide a power steering apparatus capable of preventing a driver from having a feeling of physical disorder in steering.
A power steering apparatus according to the present invention uses an electric motor as a drive source, and produces a steering assist force for assisting in steering by oil pressure created by a pump driven by the electric motor, for example. The apparatus comprises a temperature detection section for detecting the temperature in a predetermined portion; a steering angle detection section for outputting steering angle data corresponding to a steering angle; a drive inhibiting circuit for inhibiting the electric motor from being driven when the temperature detected by the temperature detection section is not less than a predetermined upper-limit temperature; a judging circuit for judging, in a state where the electric motor is inhibited from being driven by the drive inhibiting circuit, whether or not the steering angle is included in a predetermined steering angle midpoint range on the basis of the steering angle data from the steering angle detection section when the temperature detected by the temperature detection section is lowered to not more than a predetermined lower-limit temperature; and a re-drive allowing circuit for allowing, when the judging circuit judges that the steering angle is within the steering angle midpoint range, the electric motor to be driven again.
According to the present invention, when the temperature detected by the temperature detection section is not less than the predetermined upper-limit temperature, the electric motor is inhibited from being driven by the drive inhibiting circuit. When the temperature detected by the temperature detection section is lowered to not more than the predetermined lower-limit temperature in a state where the motor is inhibited from being driven, it is judged whether or not the steering, angle is within the steering, angle midpoint range. The electric motor is allowed to be driven again by the re-drive allowing circuit, provided that the steering angle is within the steering angle midpoint range.
Consequently, the assistance in steering is prevented from being suddenly started while a driver is performing a steering operation, thereby making it possible to prevent the driver from having such a feeling of physical disorder in steering that the steering resistance is rapidly lowered.
The steering angle midpoint range is a predetermined range including a steering angle midpoint, which is a steering angle in a case where a vehicle goes straight on.
Furthermore, it is preferable that the predetermined portion is the inside of a control unit for controlling the drive of the electric motor. The predetermined portion may be a motor driving element such as an FET for controlling the supply of power to the electric motor.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.